1. (Field of the Invention)
The present invention generally relates to a security sensor of a type utilizing a passive-type infrared sensor element and, more particularly, to the security sensor of a type having a disturbance detecting capability for detecting the presence or absence of an obstacle such as, for example, a transparent paint applied to the sensor casing to disable the security sensor.
2. (Description of the Prior Art)
An intruder detecting system utilizing the security sensor of the type referred to above is so designed and so configured as to detect an intruder within a detection area or an area of surveillance in reference to the difference between the temperature of a human body and the ambient temperature when the passive-type infrared sensor element receives far infrared rays of light emitted from the human body within the detection area.
It has often been experienced that the intruder detecting system is tampered with an obstacle such as, for example, a transparent paint of a kind capable of transmitting therethrough rays of light ranging from a visible wavelength region to a near infrared wavelength region, but intercepting far infrared rays of light, so that the intruder detecting system may be fooled enough to allow an intruder to trespass on the detection area monitored by the passive-type infrared sensor element. For example, while the intruder detecting system is held inoperative because the detection area is crowded with people moving in and out of the detection area, a potential intruder may enter the detection area and then apply or otherwise spray the transparent paint of the kind referred to above to a light receiving enclosure or an incident side enclosure such as, for example, a sensor lens or cover through which the far infrared rays of light enter, so that the potential intruder can enter again the detection area later while the intruder detecting system is switched in operation with the detection area no longer crowded with people.
In view of the above, the security sensor equipped with a disturbance detector for detecting the presence or absence of the obstacle has been well known in the art and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 2-287278. According to this publication, the disturbance detector used in the security sensor includes a light projecting element and a light receiving element and is so configured that while an obstacle detecting light of a wavelength ranging from a near infrared wavelength region to a visible wavelength region is emitted from the light projecting element so as to travel towards an inner surface of a lens, which forms a part of the light receiving enclosure of the security sensor and through which far infrared rays of light emitted from a human body pass onto a far infrared sensor element, the light receiving element may receive the obstacle detecting light reflected from the inner surface of the lens. In this structure, in the event that the obstacle is applied to an outer surface of the lens, the obstacle detecting light reflected form the inner surface of the lens and traveling towards the light receiving element apparently contains a component of light reflected from the obstacle and, therefore, the amount of light incident on the light receiving element is higher when the obstacle is applied to the outer surface of the lens than that when no obstacle is applied thereto. By detecting an increase in amount of the light incident on the light receiving element relative to the standard amount of light normally received by the same light receiving element, the disturbance detector can detect the presence of the obstacle on the outer surface of the lens.
It has, however, been found that with the disturbance detector used in the prior art security sensor, detection of the increment of the light reflected from the obstacle is difficult to achieve where the amount of the obstacle detecting light reflected from the obstacle is insufficiently small relative to the standard amount of the light incident on the light receiving element because the obstacle detecting light reflected from the inner surface of the lens may travel astray.
In particular, in the event that the paint of a kind capable of intercepting passage of far infrared rays of light therethrough is applied or sprayed to a front surface of the lens, the amount of light reflected from the paint decreases so extremely that the disturbance detector may fail to detect it. Moreover, since the transparent paint when applied to the front surface of the lens is virtually discernable with eyes, the presence or absence of the obstacle on the lens is not easy to detect with eyes.
In order to detect the presence of the obstacle such as the transparent paint of the kind discussed above, attempts have hitherto been made to capture an instantaneous change of the amount of the obstacle detecting light when the obstacle is applied (i.e., to detect the act of applying the obstacle) or to employ an increased emitting and receiving power of the disturbance detector. However, the former does not only require the disturbance detector to be activated at all times, but also is susceptible to an erroneous detection resulting from an erroneous operation of the disturbance detector. On the other hand, the latter may often result in an erroneous detection even when small insects traverse.
Accordingly, the present invention has been devised to substantially alleviate the foregoing problems inherent in the prior art security sensors and is intended to provide an improved security sensor having a disturbance detecting capability capable of easily detecting the presence of an obstacle such as, for example, a transparent paint of the kind referred to above when the latter is applied to a front surface of the light receiving enclosure of the security sensor.
In order to accomplish the foregoing object of the present invention, there is provided a security sensor having a disturbance detecting capability, which includes a carrier body having an infrared sensor element; an incident side enclosure or an light receiving enclosure mounted on the carrier body, said incident side enclosure comprising a lens that defines at least one detection area for the infrared sensor element or a cover that covers an incident surface area of the infrared sensor element; a light projecting element for projecting a disturbance detecting beam; a light receiving element for receiving at least a portion of the disturbance detecting beam; first and second light guide members operatively associated with the light projecting element and the light receiving element, respectively, and being cooperative with each other to define an optical path and adjacent an outer surface of the incident side enclosure or an outer surface of the carrier body adjacent the incident side enclosure so as to extend between the first and second light guide members; a detecting circuit for detecting a presence or absence of an obstacle, applied to at least one of the first and second light guide members, based on an amount of light received by the light receiving element; and surface irregularities formed on a light transmitting surface or a light reflecting surface of the first or second light guide member which surface is exposed outwardly at the optical path.
According to the present invention, when the transparent paint is applied to the outer surface of the incident side enclosure and the applied transparent paint deposits in at least some of the surface irregularities, such irregularities are filled up to define a substantially flat surface and, therefore, the amount of light incident on the light receiving element increases. Also, in the event that a black-colored paint is applied to the outer surface of the incident side enclosure and the applied black-colored paint deposits on the light transmitting or reflective surfaces of the first or second light guide member, the amount of light incident on the light receiving element decreases. Accordingly, the presence of the obstacle such as the transparent paint or the black-colored paint intercepting the far infrared light, but transmitting the disturbance detecting light can be assuredly detected. Also, even though a small obstacle such as a fly or an insect perches temporarily on the outer surface of the incident side enclosure, and since the amount of the light reflected from such small obstacle is small, there is no possibility of the security sensor functioning erroneously.
In a preferred embodiment of the present invention, the first light guide member has a first light incident surface, a first light exit surface and a first light reflecting surface defined therein and is operable to guide the beam from the first light incident surface towards the first light exit surface through the first light reflecting surface, and the second light guide ember has a second light incident surface, a second light exit surface and a second light reflecting surface defined therein and is operable to guide the beam from the second light incident surface towards the second light exit surface through the second light reflecting surface. The surface irregularities are preferably formed on at least one of the first light exit surface, the first light reflecting surface, the second light incident surface and the second light reflecting surface.
Also, in a preferred embodiment of the present invention, the first and second light guide members are mounted on the carrier body at respective locations outside an area of incidence of infrared light on the infrared sensor element. This is particularly advantageous in that the use of the first and second light guide members will not lower the detecting capability of the security sensor.
Preferably the infrared sensor element, the light projecting element and the light receiving element may be mounted on a common circuit board. This is particularly advantageous in that since only one circuit board is sufficient, the structure of the security sensor can be simplified.
Also preferably, the carrier body comprises a support base for supporting the infrared sensor element, the light projecting element and the light receiving element, a casing and a lens which defines the incident side enclosure and fitted to the casing with the infrared sensor element, the light projecting element and the light receiving element being covered by the casing and the lens. The first and second light guide members may then be positioned on the casing at respective locations adjacent the lens. This arrangement is directed to the security sensor of a type wherein the lens exposed to the outside is used to cover frontward of the detecting elements.
Again, the carrier body may comprise a support base for supporting the infrared sensor element, the light projecting element and the light receiving element, and a cover which defines the incident side enclosure and fitted to the support base so as to enclose the infrared sensor element, the light projecting element and the light receiving element, and wherein the first and second light guide members are positioned on the cover at respective locations spaced a distance from each other. This arrangement is directed to the security sensor of a type wherein the cover is used to enclose the circuit boards including the infrared sensor element.